1. Field of the Invention
The present invention generally relates to a magnetic recording medium, and more particularly, to a thermally stable high density magnetic recording medium that makes low noise.
2. Description of the Related Art
Developments in information technology require an increase in recording density of magnetic recording devices used for computer peripheral storage systems. One of the important properties required of magnetic recording media is a high S/Nm ratio (a signal to media noise ratio).
It is known that the pulse width Pw50 of a readback waveform of a general horizontal magnetic recording medium is related to a coercive force Hc, a remanent magnetic flux density Br, and a magnetic layer thickness t as follows:a∝(t×Br/Hc)1/2, andPw50=(2×(a+d)2+(a/2)2)1/2,where d is a magnetic spacing.
The pulse width at 50% amplitude Pw50 is desired to be as small as possible in order to increase the resolution of a readback signal. Accordingly, it is desirable to design a magnetic recording medium having a smaller magnetic layer thickness t and a larger coercive force Hc.
To achieve a reduction in media noise, the size of magnetic grains can be reduced and intergranular magnetic interactions can be weakened. Some methods that make magnetic grains small by adding, for example, tantalum Ta, niobium Nb, boron B, or phosphorus P, to a CoCr-based alloy are proposed.
It is further known that the addition of platinum Pt to a CoCr-based alloy, for example, increases the coercive force Hc of a magnetic layer.
Furthermore, adding Cr to the magnetic layer of a CoCr-based alloy is reported to be effective in reducing intergranular magnetic interactions in the magnetic layer. However, it is known that, if a large quantity of nonmagnetic material is added to a Co-based alloy, the in-plane orientation in the direction of the easy axis of magnetization, the hcp-C axis, is weakened.
It is reported that providing a Co-based alloy intermediate layer having a stronger in-plane orientation between the magnetic layer and an underlayer solves the problem by enhancing the in-plane orientation (as reported by S. Ohkijima et al, Digests of IEEE-Inter-Mag., AB-03, 1997, for example).
The related art in the Japanese laid-open patent application 2000-251237 discloses that the coercive force is improved by adding metal such as Pt to a CoCr-based alloy formed as an intermediate layer.
The conventional magnetic recording medium, however, still has the following two problems.
The first problem relates to an intermediate layer. The intermediate layer exhibits properties of the in-plane orientation being weakened as the quantity of nonmagnetic material increases. Accordingly, the in-plane orientation is enhanced by providing an intermediate layer made of Co-based alloy with a smaller quantity of nonmagnetic material such as Cr, Ta, Nb, B, Mn, Re, and Pt.
If the quantity of nonmagnetic material is reduced, however, a saturation magnetic flux density Bs of the intermediate layer is increased and an interfacial exchange combination between the intermediate layer and the magnetic layer is strengthened. Moreover, the magnetic layer thickness t may be kept relatively thin. A magnetocrystalline anisotropy field Hk is reduced consequently. This decrease in the magnetocrystalline anisotropy field Hk makes the magnetic recording medium thermally unstable. The high saturation magnetic flux density Bs of the intermediate layer increases intergranular interactions among magnetic grains and media noise in the transitional region.
The second problem relates to the magnetic layer. As mentioned previously, one effective approach to the reduction of the media noise is to decrease the size of the magnetic grains in the magnetic layer. However, the reduction of the magnetic grain size results in another problem, the recording destruction due to thermal instability, since the volume of magnetization per bit is also reduced. The thermal instability is controlled by increasing the quantity of Pt in the magnetic layer because the magnetocrystalline anisotropy field Hk is also increased. However, the increase in the Pt density increases the intergranular interaction, and consequently increases the media noise.
The ratio of an isolated pulse signal to a media noise (Siso/Nm) is decreased to a desired level by increasing Cr density. If Pt density is increased instead of the Cr density to increase the magnetocrystalline anisotropy field Hk, the Siso/Nm cannot be decreased enough.
FIG. 1 is a graph of the coercive force Hc of the magnetic layer of CoβCrαPt8B3 ((α=20-25 at %, β=100−(8+3+α)) as a function of the Cr density of the magnetic layer. FIG. 1 shows that the coercive force Hc of the magnetic layer depends on the Cr density in the magnetic layer.
As the Cr density increases, the media noise is reduced. But one may notice from FIG. 1 that the coercive force Hc decreases. The decrease in the coercive force Hc is caused by the decrease in the decrease in the magnetocrystalline anisotropy field Hk. This means that the magnetic recording medium becomes thermally unstable. According to the conventional technique, it is difficult to manufacture magnetic recording medium with low noise and high thermal stability.